I got some insight into what's great about Haskell by reading James Clark's "An algorithm for RELAX NG validation":

http://www.thaiopensource.com/relaxng/derivative.html

This document describes an algorithm for validating an XML document against a RELAX NG schema. This algorithm is based on the idea of what's called a derivative (sometimes called a residual). It is not the only possible algorithm for RELAX NG validation. This document does not describe any algorithms for transforming a RELAX NG schema into simplified form, nor for determining whether a RELAX NG schema is correct.

We use Haskell to describe the algorithm. Do not worry if you don't know Haskell; we use only a tiny subset which should be easily understandable.